Thermoelastic shape memory motor elements can consist of wire, ribbon, rod, coils, beams or other geometric shapes of a material such as Cu-Zn-Al or NiTi which undergo a change in shape, length or volume as a function of thermally and/or stress-induced phase changes. The most well known of these materials is Nitinol, and it has been used in thermomechanical motors and transducers of all sorts.
In heat engine applications where a Nitinol element, for example, is used to convert heat energy into mechanical work, the shape memory element is cooled to below a critical phase transformation temperature (M.sub.f) where the alloy undergoes a crystalline phase transformation into a martensitic phase. This martensitic phase is mechanically weak, and is easily deformed (superplastically) by a relative low externally applied stress. Deformations on the order of three to eight percent strain are typical. When the strained motor element is now heated above the material's austenitic finish, temperature (A.sub.f), the alloy transforms into its parent or austenitic phase with the attendant return of the element to its original length or shape.
Significant mechanical work may be performed by such shape memory materials as they recover strain in the austenitic phase. The cooling-straining and heating-strain recovery cycles may be repeated a great number of times if the maximum permissible stress and strain limits for the specific material are not exceeded. U.S. Pat. 4,197,709 describes the use of a stress-limiting system wherein the maximum permissible stress in a given shape memory element is automatically limited to a safe value. Such a stress limiter is only applicable to the whole motive shape memory element or group of elements; that is, only the total output strain is limited and not the local strain along the element.
In order to use the shape memory effect (SME) of a SME alloy to best advantage in a thermally activated transducer, the motor element must be given a shape memory anneal. This heat treatment establishes the memory shape that the element will assume when heated above the austenitic finish temperature (A.sub.f). Above the A.sub.f temperature the SME alloy is transformed into the parent austenitic phase.
The annealing process is important because all SME materials known to date do not exhibit significant or useful SME behavior in the cold-worked state. The highly cold-worked state is due to the metallurgical forming processes employed in reducing an ingot or bar of SME alloy to useful shapes such as strip, sheet, wire, tubing, etc. These forming procedures, even with intervening annealing to prevent brittleness, yield SME materials with heavily deformed martensitic structures which do not deform superplastically and are unable to transform to the parent phase with ease.
Virtually, all existing procedures for obtaining useful SME response from rolled or drawn alloys involve heat treatment (annealing) at certain carefully controlled temperatures after the final reduction in area. For example, NiTi (Nitinol) exhibits excellent shape memory response when heat-treated (memory annealed) at between 300.degree. C. to 600.degree. C., depending on the desired A.sub.f temperature.
The as-drawn or as-rolled state which typically represents a last step reduction in area of five to twenty percent, is characterized by its high tensile yield strength and low elastic modulus. Careful repeated cold working of Nitinol, for example, has yielded material with tensile yield strengths in excess of 200,000 p.s.i. with an elastic modulus of about 4.times.10.sup.6 p.s.i. while still in the martensitic phase. Such material makes excellent springs, and in fact can store significantly more energy than steel because of the eight-fold reduction in modulus of elasticity with no sacrifice in yield strength.
As used in existing shape memory effect (SME) transducers the element of SME material is annealed or heat-treated to include the whole length (La) of motor element. Typically, a long length of material is furnace-annealed, and suitable sections are then cut and mounted as required. The important point is that the full length of material, including the material held in the anchor or clamp zone, is capable of shape memory response. Starting at, for example, a temperature where the SME element is fully martensitic (below the M.sub.f), the wire of recovered length La may be strained by an applied force (F) by .DELTA.1 to a new length Ls. The actual strain .DELTA.1/La is, of course, chosen so that it does not exceed the maximum permissible strain (four to six percent for NiTi or Cu--Zn--Al) and full strain recovery is possible upon heating above A.sub.f.
The SME element (in this case a wire) heats at a rate proportional to the temperature difference of the applied heat (Hi) and the temperature of the element at any given time. Other factors affecting heat transfer to the wire and the consequent rise in temperature of the wire are well known. Thermodynamics and heat transfer engineering clearly relate the rise in temperature of the SME wire element to the Reynolds number of the surrounding fluid, surface roughness of the wire, enthalpy of the medium, differential temperatures, sensible and latent specific heats, thermal conductivity and diffusivity and other more subtle parameters. Other than controlling the specific rate of temperature rise, all these other thermodynamic considerations do not materially affect the behavior of the SME element to a rise in temperature above the A.sub.f temperature of the SME alloy. Namely, when the A.sub.s temperature is reached, the martensitic phase in the alloy transforms to the parent austenitic phase with an attendant strain recovery (.DELTA.1) to the original length (La). Work performed by the recovery strain may be extracted at the free end of the wire shown.
At the anchor (fixed) region of the wire, the SME material is buried within a structure consisting of the clamp, screw or similar mechanism. The rise in temperature of the SME material within this structure, and somewhat outboard of the actual attachment area, will lag in time with respect to the rest of the SME element. The reason for this is that direct contact with the heat transfer fluid is prevented at the attachment end(s). The added thermal mass of the anchoring mechanism prevents the SME element from uniformly heating along its full length in a simultaneous manner.
If the SME element is not required to perform any work, thereby keeping the stress in the element minimal, the effect of this uneven rise in element (wire) temperature is of little consequence. However, should the free end of the wire be constrained in any way during the shape recovery period (A.sub.s .fwdarw.A.sub.F) in order to do mechanical work, the stress in the wire will rise to some value (normally controlled to a safe maximum) for the austenitic (parent) phase, typically 30 k.s.i. to 70 k.s.i. for Niti.
Now an interesting phenomenon is observed: the average strain in the element as the A.sub.s temperature is reached is, of course equal or less than the original .DELTA.1 strain which was introduced below M.sub.f (typically four percent). However, the local strain may be much higher. In fact, local strains may be great enough (over ten percent) to severely, plastically strain the SME element in a relatively small region, usually the attachment point where the temperature rise lags behind the rest of the element. The lag in temperature of the SME material at the attachment region is due to the simultaneous lagging in time of the transformation from the weak martensitic phase to the strong austenitic phase.
The area of the SME element which remains martensitic while the rest of the element is converting to austenite has an elastic modulus of nominally 3.5.times.10.sup.6 p.s.i. compared to the 13.times.10.sup.6 p.s.i. modulus of the austenitic region. Remembering that the stress in the element is nominally equal along its length (at least for an element of constant cross section), the lower modulus area must strain more (actually three times more) on a local basis. Since the stress required to strain the martensitic area of the SME element is only a fraction of the austenitic recovery stress, permanent plastic deformation in the cooler area results.
The "lagging" portion of a typical element is restricted to areas of attachment, so that even large plastic local strains affect a real length of only several millimeters. However, the nonrecovered strain, especially in physically short elements (in actuators for example) is significant, and can complicate practical applications. Often, the nonrecoverable strain resulting from uneven heating may be compensated for by slack takeup mechanisms; however, if the local strain is very high (over ten percent) local reduction in area of the element may result with the added problem of increased brittleness due to work hardening. Eventual breakage of SME elements results and most often at the points of attachment.
For mechanical testing purposes, a heated grip has been developed wherein the area of attachment is artificially kept at a temperature above A.sub.f, so that the SME material cannot undergo excessive strain or breakage in or at the attachment area. The practical application of heated connections is somewhat limited; only certain heat engines could usefully incorporate such attachment schemes.